This invention relates to a multistage bit with several drilling stages for drilling a hole of a selected diameter, and a stopper with which drilling at a selected drilling stage can be made when said multistage bit is used.
In a prior art device, as shown in FIG. 7, there has been provided a multistage bit with several drilling stages and having a base part A whose whole profile is a stepped conical body with its tip cut off, including a longitudinal groove 1 extending from the large diameter part to the small diameter part of the stepped conical body, and a blade B directed forward of the rotating direction and downward and projecting outward in the radial direction and from the lower end of each drilling stage. The blade B of each stage is formed on the periphery 2 on the rear side in the rotating direction of the groove 1a on which the longitudinal groove 1 of each stage is formed. This prior art multistage bit has been utilized for drilling a steel plate or the like having a plate thickness less than one stage, and wherein the drilling of the hole size corresponding to one of the stages can be performed by using a respective one of several drilling stages ranging from the stage 2a of smallest diameter to the stage 2x of largest diameter, with this single bit.
In particular, in the case of this prior art multistage bit, the shank part 3 to be set on the drilling machine side (drilling machine, portable electromotive drill, etc.), the above base part A, and the tip drill 4 of the smallest diameter at the tip have been integrally formed in one piece.
And, in case of the actual drilling, for example, in case of carrying out drilling using the fourth stage from the tip, the procedures are such that the shank part 3 is set to the drilling machine, and drilling is sequentially, first with the tip drill, then with the second stage, and next with the third stage, and thereafter with the desired fourth stage.
For this reason, the tip drill and the blade part of the stage on the tip side have a higher frequency of use, and accordingly, the tip drill and the blade part of the stage on the tip side are subject to much more abrasion and damage than the other parts.
As a result, because of the fact that, contrary to the slight abrasion of the blade part of the stage near the shank part, the blade part of the stage on the tip side shows significantly more abrasion or damage, the situation makes it necessary to scrap the whole multistage bit.
Especially where the tip drill has been damaged immediately after the commencement of the drilling work using the new multistage bit, there may occur such inconvenience that the multistage bit must be scrapped without having used any blade on the rest of the stages. Also, due to the frequent occurrences of breakage on the tip drill side having smaller diameter, this defect has provided a significant problem in production efficiency.
The present invention has been made in view of the prior art problems as above indicated, and therefore it is an object to provide a multistage bit which is partially replaceable, and a stopper which is best suited to such a multistage bit.